Aging of the immune system has significant impact on the efficacy of vaccination. Immune competence starts to be compromised after the age of 50 in healthy individuats and earlier in patients with autoimmune diseases or a history of chemotherapy. Most vaccination strategies are developed in young adults and need to be optimized to meet the needs of the middle-aged or elderly immune system. We hypothesize that the dominant mechanisms generating the clinical picture of immune senescence are (1) a reduced ability of dendritic cells to induce T-cell responses, (2) a contraction of the diversity of the T-cell receptor repertoire, and (3) cellular senescence of naive and memory T cells. With the decline of thymic function, the repertoire of naive T cells contracts, restricting the availability of T-cell receptors that have an optimal fit for pathogenic antigens. Naive and memory T cells are under replicative stress, both from antigen-specific and from homeostatic proliferation. As a consequence, telomeres are eroded with age, limiting the ability of T cells to respond with clonal expansion. Senescence of memory T cells has been associated with changes in gene expression that are detrimental to memory cell function, such as loss of CD28 and the gain of killer cell immunoglobulin-like receptors (KIRs). Specific aims have been designed to identify the immunological pathways that are compromised and to develop strategies to overcome these deficits. Specific Aim 1 will focus on primary T-cell responses, memory T-cell responses are examined in Specific Aim 2. We will correlate primary T-cell responses in an in vitro system with the age of the donor, and we will use functional assays as well as genomic and proteomic studies to identify pathways that correlate with decreased responsiveness or proliferative arrest of naive T cells. These pathways will be targeted to improve primary immune responses. We will test the relevance of these observations in vivo and determine whether the repertoire of T cells induced by anthrax vaccination is age-dependent. In Specific Aim 2, we will use functional assays and genomic and proteomic studies to identify defects in memory cell responses. CD28 loss, KIR expression and other newly identified pathways will be targeted to overcome these defects. Additional studies in vivo will determine how age-dependent mechanisms influence T memory cell homeostatic proliferation, long-term persistence of vaccinia-specific memory cells and the ability of memory cells to respond to antigenic rechallenge in vivo. Taken together, these studies will develop strategies to improve primary and recall responses in middle-aged and elderly individuals to vaccination.